The present invention relates generally to lasers and more particularly to a laser in which the laser medium is a single crystal of chromium-doped beryllium aluminum silicate (Emerald).
The use of lasers in science and industry has received wide acceptance in an ever increasing variety of applications. Lasers have found use in such diverse areas as range finding apparatus, optical surgery, optical printers, optical readers and metal drilling. Briefly, lasers operate on the principle of light amplification through stimulated emission of radiation and can create extremely intense concentrations of light. The coherent light beam produced in a laser cavity is amplified in a laser material. Materials which have been used as laser hosts include gases, liquids, glasses and single crystalline solids.
When single crystalline solids are utilized in lasers, the crystals are generally in the form of elongated rods. The structure of the crystalline material must be very nearly perfect, since any optical inhomogeneities will cause distortion and scattering of the laser beam and thereby reduce the intensity and coherence of the radiation. Imperfections in the crystal which adversely affect lasing performance include elastic strain, crystal misorientations, chemical concentration inhomogeneities, dislocations, inclusions and bubbles.
The first room temperature single crystal solid laser, disclosed several years ago, utilized Al.sub.2 O.sub.3 Cr.sup.3+ (Ruby). In more recent developments, room temperature lasers have been fabricated from rare earth ion-doped materials. Examples include Y.sub.3 Al.sub.5 O.sub.12 :Nd.sup.3+ (YAG:Nd) and Y.sub.2 Al.sub.2 O.sub.6 :Md.sup.3+ (YALO:Nd).
During the past ten years, ultrafast (i.e. picosecond) laser pulses have been generated in the visible and infrared spectral region by passive and active mode-locked Ruby, neodynium and dye laser systems. The current solid state picosecond laser systems are not tunable and the pulse duration is limited by the spectral characteristics of the lasing material. There is a need for high power tunable picosecond laser source. The availability of high intensity, tunable, subpicosecond pulses can be used to investigate a variety of phenomena such as optically generated plasmons, optical radar, optical computers, high speed photography, optical and vibrational spectroscopy of liquids, solids and gases, semiconductor properties, nonlinear properties of materials, transient responses of optical and electronic instruments.
The types of materials commonly used in the solid state laser are in the form of glasses (such as silicate or phosphate) or crystals doped with such ions as Cr.sup.3+ or Nd.sup.3+. As is known, there are certain conditions that the laser material must fulfill to generate short pulses, namely, wide fluorescence bandwidth, high quantum yield, and strong and broad absorption bands in the visible. Ruby is a three level material (Sapphire Al.sub.2 O.sub.3 in which a small percentage of Al.sup.3+ has been replaced by Cr.sup.3+) which lases in the visible at 694.3 nm. The bandwidth, however, is very narrow i.e. about 0.5 .ANG., due to the crystalline structure of the material (lasing at R lines). This sets a lower limit to the pulse duration of 20-30 picoseconds. The other commonly used doping material, Nd.sup.3+, is generally either in crystalline host like Yttrium Aluminium Garnet (YAG) or in glass (silicate or phosphate). All three Nd.sup.3+ doped materials (YAG, silicate and phosphate) lase in the infrared around 1060 nm with durations of 30, 8 and 6 picoseconds, respectively. Dye laser systems (laser systems in which the laser material is an organic dye) generally lase in visible and can generate subpicosecond pulses around 600 nm. (for Rhodamine 6G). However, the very low power emitted from such lasers require the use of a complicated amplifier system. In addition, dye lasers essentially cannot be Q-switched.
In U.S. Pat. No. 3,997,853, to R. C. Morris et al there is disclosed a laser in which the host comprises a single crystal of beryllium aluminate (BeAl.sub.2 O.sub.4) doped with trivalent chromium ions, the single crystal being crystallographically oriented substantially along the a-c plane, at least 30.degree. removed from the b-axis, and having a chromium doping concentration ranging from about 0.005 to 1.0 atom percent.
In U.S. Pat. No. 4,272,733, to J. C. Walling etc., there is disclosed a high power, broadly wavelength-tunable laser system which comprises as the laser medium particular single crystals of chromium-doped beryllium aluminate (BeAl.sub.2 O.sub.4 :Cr.sup.3 +) having the chrysoberyl structure, means for exciting the laser medium and tuning means. The laser may be operated over a broad temperature range from cryogenic temperatures to elevated temperatures. Elevated temperatures are preferred, however, since they result in higher laser gain. Emission is in a spectral range from red to infrared, and the laser is useful in the fields of defense, communications, isotope separation, photochemistry, etc.
In U.S. Pat. No. 4,019,156, to W. D. Fountain there is disclosed a Q-switched/mode-locked Nd:YAG laser oscillator employing simultaneous active (electro-optic) and passive (saturable absorber) loss modulation within the optical cavity is described. This "dual modulation" oscillator can produce trasform-limited pulses of duration ranging from about 30 psec to about 5 nsec with greatly improved stability compared to other mode-locked systems. The pulses produced by this system lack intrapulse frequency or amplitude modulation, and hence are idealy suited for amplification to high energies and for other applications where well-defined pulses are required. Also, the pulses of this system have excellent interpulse characteristics, wherein the optical noise between the individual pulses of the pulse train has a power level well below the power of the peak pulse of the train.
In an artical entitled Color by Kurt Nassau appearing in Scientific American, October 1980, Volume 243, Number 4, pp. 124-156, various properties of Ruby, alexandrite and Emerald are discussed.